my friend did it for two undergraduate degrees, did commerce at monash and a health related degree at UoM (pre 2007) i wont say the true name of the health degree as I do not want to disclose the persons identity.
He basically did a mixture of deferred/intermissions/summer subjects/overloading and cleverly transferred the subjects back and forth as electives/credits. he finished the single commerce in 4 years and the health degree around5-6 concurrently, so he spent a total of 6 years at universities got 2 degrees.
The reason you might be thinking, is because commerce/health field wasnt a double degree the only one that I can think of is pharmacy/commerce.
so im pretty sure you can, but postgrade degrees are probably a little bit more intense than undergad, especially jd they recommend doing trimesters and you basically wont have time to do much. Jd at UoM cannot be done part time so i wouldnt suggest it.
I wouldnt do 2 postgrads concurrently as it will cost your friend quite abit. But if your friends insist a good way to do it would be do JD unaccelerated and do a diploma in whatever field of commerce part time at whatever uni (1 unit per semester, so 3 units a year). So when your friend does finish his JD, he/she will also finish with a diploma at around the same time. A JD alone would be able to suffice most needs of corporate work, the diploma is just there to give a twist.
In my opinion, I rkn the master of commerce is not worth it if you are going for JD. JD is probably enough as it is. (UoM or Monash)